The present disclosure contemplates that reinforced concrete structures may be prestressed using “post-tension” methods and/or “pre-tension” methods. Post-tension reinforcement of concrete may create compressive forces in a concrete beam and/or plate that may be cast on-site, such as a floor of an office building or a beam of a bridge. For example, in post-tension reinforcement, concrete may be poured over greased cables that may be sheathed in plastic tubing. Although the plastic tubing may become adhered to the concrete, the grease may allow the cables to slide within the tubing. After the concrete cures, special machines and/or fixtures may be used to pull on protruding cable ends, creating tension, and/or to fix the cable ends in position while maintaining the tension on the cables. Thus, the cables may exert compressive forces on the concrete.
The present disclosure contemplates that in some pre-tension methods, a solid reinforcing bar (“rebar”) may be held in tension while concrete is poured and/or cures around the rebar. Once the concrete has cured, the externally applied tension on the rebar may be released, thereby allowing the rebar to apply compressive stresses to the concrete. Due to the difficulties of stretching rebar on-site, this method may be performed in a factory.
As used herein, “stress” may refer to a measure of the internal forces acting within a deformable body. As used herein, “strain” may refer to the deformation of a physical body under the action of applied forces.
The present disclosure contemplates that post-tensioning with cables may not be as effective for increasing the strength of concrete members as cast-in-place pre-tensioned rebar. The difference may be at least partially because compressive stress produced by a post-tension cable may not be applied evenly throughout the concrete (e.g., along the length of the cable), but instead may be applied substantially at the cable ends (e.g., on the outer surface of the concrete where the cables terminate). A post-tension cable, sliding in a greased tube, may not apply substantial forces inside the concrete structure (except, if the cable is curved, it may apply lateral forces generally at right angles to the cable). In contrast, a pre-tension rebar may apply forces to the concrete along the length of the rebar.